My thoughts on defense.

06/09/2017

Project Yehudi

In the early days of World War II, the US Navy had a problem in the campaign against German U-boats. It was fairly common for a patrol plane to spot a surfaced U-boat from a considerable distance. But very often, the U-boat would spot the patrol plane, and crash dive to safety before the patrol plane could attack.

The Navy wanted to reduce the visual detection range of its aircraft. Looking at previous Canadian research into the issue, it soon became clear that the visual detection range of aircraft from ships was due primarily to the aircraft appearing darker than the sky background. Even if an aircraft was painted white, it still appeared darker then the ambient light.

And so, the crazy way to make an airplane invisible during the day was to use bright lights. Shining bright lights on the airplane would make it blend with the ambient light, and thus reduce the contrast that made spotting it easy.

Unfortunately, shining a bright light all over an airplane wasn’t exactly practical.

But the Navy did a little more experimentation, and figured out that bright lights on the nose and the leading edge of the wing, pointing forward, would essentially make a plane invisible as long as it was pointed directly at the target, and would require far less electrical power.

In 1943, a TBM Avenger was outfitted with such an array, under Project Yehudi. The average visual detection range in tests on Chesapeake Bay dropped from 12 miles, to as little as a mile and a half.

While the system worked, the increasing use of radar in the Battle of the Atlantic meant that U-boats would increasingly stay submerged during daylight hours, thus negating the need for Yehudi lights, and so the project was essentially abandoned and forgotten.

Similarly, a squadron of B-24 bombers tested a Yehudi array to help defeat visually aimed flack. The increasing use of radar directed anti-aircraft artillery meant the tradeoff of installing the array wasn’t worth it.

A quarter century later, the concept would make a brief reappearance. The Ault Report on the Navy’s air to air performance in Vietnam noted that the F-4 Phantom was much easier to detect visually than the nimble MiG-17 and MiG-21 fighters it faced over North Vietnam. Again, a series of tests looked at using a Yehudi approach to minimizing the visual signature of Navy aircraft. While Yehudi lights were again not adopted for the fleet, the Compass Ghost paint scheme devised in concert with this project was, and is essentially the basis for all US tactical aircraft camouflage schemes.

Here’s a brief video showing how to hide an armored vehicle with the counterillumination concept.